After 1920 - that's when Savage bought Stevens.
Before, say, 1962 when the large Chicago hardware outfit Hibbard, Spencer, Bartlett & Co. was sold to John Cotter & Co. for the True Value Brand Name
Whippet was a name used on much earlier guns made for HSB by H.D. Folsom and on later guns by Savage/Stevens/Springfield. (I actually ran across an ad for a pump .22 Whippet while I was looking this up.)
I don't have a clue on value. If it was a plain old Stevens I'd say $100 give or take some based on condition. It's hard for the prices to go up much when folks can just go out and buy a new one.
John